Knowledge of a few general principles of engine operation will
help the pilot obtain increased dependability and efficiency from the engine
and, in many instances, this knowledge will help in avoiding engine failure.

In this short chapter, it is impractical to discuss in detail
the various types of engines and the finer points of operation which can
be learned only through experience. Information from the manufacturer’s
instruction manual; familiarity with the operating limitations for the
airplane engine; and specific advice from a flight instructor, combined
with the information contained within this section, should provide adequate
information to operate an airplane engine satisfactorily.

Figure 2-5.—Basic parts of a reciprocating engine.

How an Engine Operates

Most light airplane engines are internal combustion of the reciprocating
type which operate on the same principle as automobile engines. They are
called reciprocating engines because certain parts move back and forth
in contrast to a circular motion such as a turbine. Some smaller airplanes
are equipped with turbine engines, but this type will not be discussed
in this handbook. As shown in figure 2-5, the reciprocating engine consists
of cylinders, pistons, connecting rods, and a crankshaft. One end of a
connecting rod is attached to a piston and the other end to the crankshaft.

This connecting rod converts the straight-line motion of the piston
to the rotary motion of the crankshaft, which turns the propeller. At the
closed end of the cylinder, there are normally two spark plugs which ignite
the fuel, and two openings over which valves open and close. One valve
(the intake valve) when open admits the mixture of fuel and air, and the
other (the exhaust valve) when open permits the burned gases to escape.
For the engine to complete one cycle, the piston must complete four strokes.
This requires two revolutions of the crankshaft. The four strokes are the
intake, compression, power, and exhaust.

Diagram A of figure 2-6 shows the piston moving away from the cylinder
head. The intake valve is opened and the fuel/air mixture is drawn into
the cylinder. This is the intake stroke.

Diagram B shows the piston returning to the top of the cylinder.
Both valves are closed, and the fuel/air mixture is compressed. This is
the compression stroke.

Diagram C shows that when the piston is approximately at the top
of the cylinder head, a spark from the plugs ignites the mixture, which
burns at a controlled rate. Expansion of the burning gas exerts pressure
on the piston, forcing it downward. This is the power stroke.

Diagram D shows that just before the piston completes the power
stroke the exhaust valve starts to open, and the burned gases are forced
out as the piston returns to the top of the cylinder. This is the exhaust
stroke. The cycle is then ready to begin again as shown in Diagram A.

From this description, notice that each cylinder of the engine
delivers power only once in every four strokes of the piston or every two
revolutions of the crankshaft. The momentum of the crankshaft carries the
piston through the other three strokes although the diagram shows the action
of only one cylinder. To increase power and gain smoothness of operation,
other cylinders are added and the power strokes are timed to occur at successive
intervals during the revolution of the crankshaft.
Aircraft engines are classified by the various ways the cylinders
are arranged around the central crankcase. Most general aviation airplane
engines are classed as the horizontally opposed, which have the cylinder
banks arranged in two rows, directly opposite to each other and using the
same crankshaft.

Larger and more powerful reciprocating engines are classed as
radial engines. In these engines, the cylinders are placed in a circular
pattern around the crankcase, which is placed in the center of the circle.

Other engine classifications are the in-line engine with the cylinders
placed in one straight row, and the “vee” type with the cylinders placed
in two rows forming a “V” similar to the V-8 engine used in automobiles.

Cooling System

The burning fuel within the cylinders produces intense heat, most
of which is expelled through the exhaust. Much of the remaining heat, however,
must be removed to prevent the engine from overheating. In practically
all automobile engines, excess heat is carried away by a coolant circulating
around the cylinder walls.

Most light airplane engines are air cooled. The cooling process
is accomplished by cool air being forced into the engine compartment through
openings in front of the engine cowl. This ram air is routed by baffles
over fins attached to the engine cylinders, and other parts of the engine,
where the air absorbs the engine heat. Expulsion of the hot air takes place
through one or two openings at the rear bottom of the engine cowling.

Some airplanes are equipped with a device known as cowl flaps
which are used to control engine temperatures during various flight operations.
Cowl flaps are hinged covers which fit over the opening through which the
hot air is expelled. By adjusting the cowl flap opening, the pilot can
regulate the engine temperature during flight. If the engine temperature
is low, the cowl flaps can be closed, thereby restricting the flow of expelled
hot air and increasing engine temperature. If the engine temperature is
high, the cowl flaps can be opened to permit a greater flow of air through
the system, thereby decreasing the engine temperature. Usually during low
airspeed and high power operations such as takeoffs and climbs, the cowl
flaps are opened. During higher speed and lower power operations such as
cruising flight and descents, the cowl flaps are closed.

Under normal operating conditions in airplanes not equipped with
cowl flaps, the engine temperature can be controlled by changing the airspeed
or the power output of the engine. High engine temperatures can be decreased
by increasing the airspeed and/or reducing the power.

The oil temperature gauge indicates the temperature of the oil
which is heated by the engine; therefore, this gauge gives an indirect
and delayed indication of rising engine temperature. However, the oil temperature
gauge should be used for determining engine temperature if this is the
only means available.

Many airplanes are equipped with a cylinder-head temperature gauge.
This is an additional instrument which will indicate a direct and immediate
cylinder temperature change. This instrument is calibrated in degrees Celsius
or Fahrenheit, and is usually color coded with a green arc to indicate
the normal operating range. A red line on the instrument indicates maximum
allowable cylinder head temperature.

To avoid excessive cylinder head temperatures, a pilot can open
the cowl flaps, increase airspeed, enrich the mixture, or reduce power.
Any of these procedures will aid in reducing the engine temperature.

When an airplane engine is operated on the ground, very little
air flows past the cylinders (particularly if the engine is closely cowled)
and overheating is likely to occur. Overheating may also occur during a
prolonged climb, because the engine at this time is usually developing
high power at relatively slow airspeed.

Operating the engine at higher than its designed temperature can
cause loss of power, excessive oil consumption, and detonation. It will
also lead to serious permanent damage, such as, scoring the cylinder walls,
damaging the pistons and rings, and burning and warping the valves. To
aid the pilot in avoiding excessive temperatures, engine temperature instruments
in the cockpit should be monitored in flight.